There is growing evidence to suggest that using existing vaccines could be beneficial against COVID-19 — even though they are not specific to the novel coronavirus SARS-CoV-2.

Several clinical trials are taking place around the world to test whether using the BCG (Bacillus Calmette–Guérin) vaccine, which protects against tuberculosis (TB), could be effective in COVID-19.

Scientists think that the vaccine could boost a person’s immune response, reduce their levels of SARS-CoV-2, and lessen the symptoms associated with COVID-19.

A new report published in the American Society for Microbiology’s journal mBio suggests that the MMR vaccine, which is routinely administered in childhood, could serve a similar purpose.

The researchers behind the article suggest that the vaccine could dampen the severe inflammation associated with COVID-19 and mortality, and propose a clinical trial for healthcare workers.

Training the immune system

The type of vaccines that have an effect against unrelated infectious are called live attenuated vaccines. This means they contain real viruses or bacteria that scientists have weakened in a laboratory.

Studies show that these vaccines protect against other infections by ‘training’ the immune system in a non-specific way. This type of non-specific immune response is the first line of defense against infection and is called the innate immune response.

“Live attenuated vaccines seemingly have some nonspecific benefits as well as immunity to the target pathogen,” explains co-author of the new paper Dr. Paul Fidel, Jr., Associate Dean for Research at Louisiana State University Health School of Dentistry in New Orleans.

Dr. Fidel, together with Dr. Mairi Noverr, Professor of Microbiology & Immunology at Tulane University School of Medicine in New Orleans, suggest that the protection afforded by these vaccines is due to myeloid-derived suppressor cells, or MDSCs, a type of immune cell that comes from bone marrow.

Scientists have shown that these cells can reduce inflammation and mortality in mouse models of infection. In their own research, Drs. Fidel and Noverr have shown that vaccination with a live-attenuated fungus may protect against sepsis thanks to MDSCs.